It can be beneficial to move phi nodes, even though they can often be coalesced.
Model this cost so nir_opt_preamble can make good decisions about hoisting phi
nodes (and by extension, if-statements) into the preamble.
At this point in the series, this has no effect, but it will avoid certain
shader-db regressions associated with the nir_opt_preamble changes later in the
series.
Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24011>
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